Proposals for optimizing sea lamprey passage through a vertical-slot fishway

Abstract

Movement of sea lamprey through a vertical‐slot fishway (built at the Coimbra dam,in central Portugal) was continuously monitored between 2013 and 2016 by a videorecording system. Visual count data were used to quantify the overall successfulmovements and identify seasonal and circadian patterns of fishway use. Explicativemodels (Boosted Regression Trees) were used to study the relationship between frequencyof successful movements during the species migration peak and environmentalvariables. The aim of the study was to identify predictors that may be related withsuccessful sea lamprey upstream migration through the fishway. Collected informationwas used to support further management recommendations for optimizing thefishway performance. During the 4‐year study, near 50,000 lampreys successfullynegotiated the Coimbra dam fishway reaching the upper stretch of river Mondego.Migratory peak occurred between April and May with an increase in passages duringthe night period (between 11 p.m. and 6 a.m.). It was observed an increase in use frequencythat was related with the combined effect of flow and temperature. Higherpassages were achieved when Coimbra dam discharge flows are lower than100 m3/s and temperature between 15 and 19°C. Flow discharges higher than150 m3/s seemed to inhibit successful sea lamprey passage. In dry years, when flowsare almost constantly lower than 50 m3/s, temperature was the most important factorinfluencing fishway use.